I30 



NATURE 



{June 9, 1887 



commutators were locked by the nut, N, being rotated at 

 any speed by a small electromotor, not shown in the 

 figure, to which was attached a Young's speed indicator, 

 which registered the speed of rotation at any moment. 

 The brushes, Bj, Bg, b^, b.^, were fixed to the baseboard 

 and joined to the bridge, as indicated in the figure. 

 When the double commutator was rotated by the motor 

 (of which the speed was correctly adjusted by means of a 

 Varley's flexible carbon-resistance), the portion ab caused 

 the battery circuits to be periodically made and broken, 

 while the other portion, c D, periodically short-circuited 

 and unshort-circuited the galvanometer, so that the 

 following cycle of operation, called for simplicity 07te 

 operation, was performed any desired number of times 

 per minute : — 



Battery circuit. 



Make. 



While made. 

 Break. 



While broken. 

 Make. 



Galvanometer short circuit. 



While broken. 

 Make. 



While made. 

 Break. 

 While broken. 



If we call 



angle batweea the slits in the two commutators 

 360^ 



the lead or /, so that / will be equal, for example, to \ when 

 each of the cycle of operations given in the table lasts for 

 one-quarter of a revolution, then we have shown that 



L = -o- second- ohms, 



71 



where 71 is the number of revolutions of the commutator 

 per second, and a the apparent increase of resistance of 

 the coil with self-induction, or 



T 60/cr J 1 



L = second-ohms 



N 



where N is the number of revolutions per minute. 



A number of experiments were made in the summer 

 and autumn of last year, and they showed that this new 

 method was very accurate and furnished an extremely 

 sensitive test for the absolute measurement of a small 

 coefficient of self-induction. 



By the simple addition, therefore, of such a com- 

 mutating arrangement as we have described to an ordin- 

 ary Wheatstone's bridge, it becomes possible, whenever 

 the resistance of a coil electro-maget, &c., is being 

 measured, to measure also the coefficient of self-induction 

 in absolute measure, by a zero method which is as 

 sensitive for the measurement of self-induction as the 

 ordinary Wheatstone's bridge method is for the measure- 

 ment of resistance. 



The instrument previously described requires an electro- 

 motor to drive it, and a speed-indicator to register its 

 speed, hence it would be too cumbersome for every-day 

 work. It therefore became necessary to devise com- 

 mercial apparatus, and this was done as follows : — 



Attached to the commutator of our self-induction 

 apparatus is a box, B (Fig. 2), fitted with weighted elastic 

 sides made of corrugated steel, which fly out more and 

 more, under the action of centrifugal force, as the box is 

 rotated faster and faster. A stout glass tube, G G', of com- 

 paratively small bore, open at both ends, is cemented 

 into a collar in the axis of the box, and rotates with the 

 box. The box is completely filled with mercury, and the 

 tube partially, hence when the volume of the box expands 

 as its sides fly out the length of the column of mer- 

 cury in the tube diminishes, and the length of the 

 column at any moment is a measure of the speed of rota- 

 tion of the box. In the neck of the collar, c, in which 

 the tube is cemented, there is a steel tap attached to an 

 axial spmdle passing through a tube inside the box, and 



projected out of this tube at the other end of the box. If 

 this spindle be turned relatively to the box, the tap is 

 opened or closed. At the commencement of the experiment 

 the tap is opened, and the handle, H, is turned with the 

 right hand, faster and faster, until, on depressing the key, K, 

 with the left hand from time to time, the galvanometer 

 needle is seen to be approaching zero, or the spot of 

 light the zero position on the scale The key may now 

 be kept depressed, and on turning the handle a little 

 faster a speed is at length reached producing exact 

 balance — if the handle be turned faster, the needle or 

 spot of light deflects to one side of the zero, if more 

 slowly to the other — at this moment the trigger, T, is 

 lightly touched with the left hand, and a spring is 

 liberated. This has the effect of producing a resistance 

 to the rotation of the tap-spindle, which previously was 

 rotating freely with the rotating box, and the tap is thus 

 turned off, cutting off the connexion between the mercury 

 in the glass tube and t'lat in the box. Consequently the 

 mercury in the tube remains, even after the instrument 

 is stopped, of exactly the same length that it had when 

 the trigger was touched. The position of the end of 

 thread of mercury in the tube is now read off on the scale 

 attached, and the apparent increase of resistance of the 

 coil, electromagnet, or whatever it may be, divided by 

 the number on the scale, gives the required coefficient 

 of self-induction in second-ohms without any further 

 calculation. 



The instrument is, therefore, direct-reading. 



At first, rotating commutators similar to those shown 

 in Fig. I were employed with the apparatus shown in 

 Fig. 2 ; next the brushes were made of a variety of 

 different forms, so as to press radially on the rotating 

 commutators to prevent the wear altering the lead, and 

 thus changing the sensibility of the instrument ; but this 

 form of commutator has at length been entirely super- 

 seded by the two oscillating arms, or brushes. A, A, worked 

 by a cam. Each arm is composed of several pieces of 

 hard copper, contact being made through the ends, as in 

 many of the switches now used for electric-light work. 

 The end of each brush alternately rubs on a flat piece of 

 phosphor bronze, P, P, when it makes contact, and on a 

 flat piece of glass or agate, g^ when it does not. This 

 form of commutator we found superior for our purpose to 

 the double cylindrical one, since, with the two oscillating 

 arms, the lead can be more easily varied for adjustment ; 

 and this slight adjustment of the lead, we may here men- 

 tion, forms the fine adjustment in the construction of this 

 direct-readmg instrument. Further, the slow wearing of 

 this form of brush does not alter the lead ; consequently 

 the value of the graduations of the scale remains 

 constant. 



The temperature adjustment of the instrument is 

 effected by moving the scale until the zero is opposite the 

 end of the thread of mercury when the instrument is at 

 rest. 



Following the precedent of naming an instrument after 

 the name of the unit employed — for example, "ammeter," 

 " voltmeter," "' ohmmeter," " wattmeter," — it seems desir- 

 able to call this instrument after the name of the com- 

 mercial unit of self and mutual induction. The absolute 

 electro-magnetic unit of self and mutual induction is i 

 centimetre, a name used by all scientific nations. But 

 the commercial unit of self and mutual induction is 

 99,777 X 10* centimetres, or the second-ohm, which is 

 about 2'3 in a thousand less than 10" centimetres, or one 

 earth's quadrant. . Now, in spite of the difference between 

 these two numbers, which, although small, it is a pity to 

 lose sight of, the English word " quadrant " is not used 

 in French, therefore it would not be well to suggest this 

 word as the international name for the unit. Yet it is 

 most important that some name should be universally 

 adopted, since the use of simple familiar names has much 

 to do with making people familiar with the laws of the 



